1. Field of the Invention
The present invention relates to a color filter substrate and a method for fabricating the same, and more particularly, to a color filter substrate and a method for fabricating the same that includes spacers and a black matrix.
2. Description of the Related Art
Until recently, display devices generally employed cathode-ray tubes (CRTs). Presently, many efforts are being made to study and develop various types of flat panel displays. For instance, thin film transistor-liquid crystal displays (TFT-LCDs) have been developed as substitutions for CRTs because of their high resolution images, lightness, small thickness, compact size, and low voltage power supply requirements.
FIG. 1 illustrates a layout of a liquid crystal display (LCD) device according to the related art. In FIG. 1, the LCD device 11 includes an upper substrate 5, a lower substrate 22, and a liquid crystal material layer 14 interposed between the upper and lower substrates 5 and 22. A black matrix 6, a color filter 8, and a transparent common electrode 18 are formed on the upper substrate 15, such that the transparent common electrode 18 is formed on the color filter 8. The color filter 8 includes sub-color filters in color red (R), green(G), and blue (B), wherein the upper substrate 5 is commonly referred to as a color filter substrate. In addition, pixel regions P, pixel electrodes 17, gate lines 13, data lines 15, and switching devices T are formed on the lower substrate 22, such that the pixel electrodes 17 are formed within the pixel regions P and the switching devices T are formed at intersections of the gate and data lines 13 and 15. Moreover, the lower substrate 22 is commonly referred to as an array substrate or a thin film transistor substrate, wherein the switching devices T are arranged in a matrix configuration. The switching device T may include thin film transistors (TFTs) to switch an arrangement direction of liquid crystal molecules within the liquid crystal material layer 14. The pixel regions P are defined by the gate and data lines 13 and 15, and a transparent conductive metal, such as indium-tin-oxide (ITO) having a relatively high transmittance, is used to form the pixel electrodes 17 within the pixel regions P.
FIG. 2 illustrates steps for fabricating a thin film transistor substrate of a liquid crystal display device according to the related art. In FIG. 2, at step st1, a thin film transistor substrate is prepared by forming an array of thin film transistors and pixel electrodes on a lower substrate. At step st2, an orientation layer is formed on the lower substrate by depositing a polymeric material thin film on an entire surface of the lower substrate, and performing a uniform rubbing process. Generally, an organic material, such as polyimide, is used as the polymeric material thin film for forming the orientation film. During the rubbing process, the orientation layer is rubbed with a cloth along one direction, thereby aligning the liquid crystal molecules within a liquid crystal material layer (to be inserted later) along an initial array orientation corresponding to the rubbing direction, thereby providing normal operation of the liquid crystal molecules and uniform display characteristics.
At step st3, a seal pattern is formed through a printing process. The print process includes forming a desired pattern by screen-printing a thermosetting plastic material. The seal pattern serves two functions: forming a gap for a subsequent liquid crystal material injection process and providing confinement of the liquid crystal material.
At step st4, spacers are formed through a spraying process. The spacers maintain a precise and uniform gap between the lower substrate and an upper substrate, wherein the size of the spacers determines the dimension of the gap. Accordingly, the spacers are sprayed evenly onto one of the upper and lower substrates. Two different types of spraying process can be employed. First, a wet spraying process sprays a mixture of alcohol and a spacer material. In addition, a dry spraying process sprays only a spacer material. Moreover, the dry spraying process involves an electrostatic spray procedure that uses electrostatic forces or a non-electric spraying procedure that uses gas pressures. The non-electric spraying procedure is generally used when static electricity can damage the liquid crystal structure, and the procedure include electrifying a predetermined amount of spacer particle by friction through pipes, and releasing the spacer particle from a nozzle to the substrate. In addition, the electrostatic spraying procedure generally includes releasing a spacer particle from a high-voltage-applied nozzle to an grounded substrate.
At step st5, an attachment of the upper substrate (color filter substrate) and the lower substrate (thin film transistor substrate) is made through an attaching process. The margin of accuracy for attaching and aligning the upper and lower substrates is determined based on the design of each substrate. In general, accuracy within a few micrometers (μm) is required. If the accuracy of the alignment is not within the margin, the liquid crystal material layer will not properly operate due to light leakage.
At step st6, a cutting process of the liquid crystal cell is performed to create individual unit cells. For example, a number of individual unit cells are formed on one large-sized glass substrate, and the large-sized substrate is subsequently divided into the individual unit cells through the cutting process. Although the cutting process can be performed after the liquid crystal material is injected, the liquid crystal material is commonly injected into the individual unit cells because the size of the large-sized substrate. The cutting process includes a scribing process that forms cutting lines on a surface of the large-size substrate using a diamond pen, and a breaking process that divides the substrate along the cutting lines by application of force.
At step st7, the liquid crystal material is injected between the upper and lower substrates of each individual unit through a injection process. Typically, each of the individual unit cells has a size of several hundreds of square centimeters with a gap of several micrometers between the upper and lower substrates. Accordingly, a vacuum injection procedure is commonly used.
Alternatively, spacers can be formed through a patterning process, rather than the spraying process. For example, the patterning of spacers may be performed on the lower substrate (the thin film transistor substrate) over the matrix structure, or the patterning of spacers may be performed on the upper substrate (the color filter substrate) by using a photosensitive material.
FIGS. 3A-3G illustrate a method of forming a color filter substrate of a liquid crystal display device according to the related art. In FIG. 3A, a photosensitive black organic material is deposited on a transparent insulating substrate 5, thereby forming a black organic layer 4. The photosensitive black organic material can be a positive type where portions that are subsequently exposed to light are removed by a development process, or a negative type, such that portions that are subsequently exposed to light are not removed by a development process. In addition, a mask 19 having light-transmitting portions A and light-shielding portions B is disposed over the black organic layer 4. Subsequently, light L1 irradiates portions of the black organic layer 4 through the light-transmitting portions A of the mask 19. In FIG. 3B, after developing the light-exposed black organic layer 4, a black matrix 6 is formed on the transparent insulating substrate 5. Generally, the black matrix 6 is formed between red/green/blue patterns (sub-color filters) to screen light along a boundary of pixel electrodes 17 (in FIG. 1). The black matrix 6 is commonly formed of a metal thin film, such as chromium (Cr), a carbon-based organic material having an optical density of more than 3.5, or a double layer structure of Cr and chromium-oxide (CrOx), to form a uniform lower reflection layer. The specific material used for forming the black matrix 6 is commonly based on the material availability.
In FIG. 3C, a color filter using a color resin of red (R), green (G), and blue (B) is formed on the substrate 5 and the black matrix 6. The color resin includes a photo polymerization initiation material, a photo polymerization type photosensitive composition material, and an organic pigment that has red/green/blue or similar colors. For example, a red color resin is first deposited on an entire surface of the substrate 5, and a red sub-color filter 8a is formed within a desired region by selective exposure of light. Next, a green color resin is deposited on the entire surface of the substrate 5, and a green color filter 8b is formed by another selective exposure of light. Then, a blue color resin is deposited on the entire surface of the substrate 5, and a blue color filter 8c is within a desired region by another selective exposure of light. Although the above-discussed sequence of the color resin is red, green, blue, any sequence of color resin may be chosen. Accordingly, the substrate 5 can now be referred to as a color filter substrate.
In FIG. 3D, after sub-color filters 8a, 8b, and 8c are formed on the substrate 5 and the black matrix 6, a planarization process is performed. The planarization process includes depositing a transparent resin having insulating properties on the color filter substrate 5 to form an overcoat layer 26.
In FIG. 3E, a process of forming an electrode on the color filter substrate 5 is performed. In general, when using the color filter substrate 5 as the upper substrate of the liquid crystal display panel, a transparent electrode 18 is formed on top of the color filter substrate 5. The transparent electrode 18 (i.e., the common electrode) will be used to drive the liquid crystal material layer 14 (in FIG. 1) with a pixel voltage. The common electrode 18 is formed by depositing a transparent conductive material, such as indium-tin oxide (ITO) or indium-zinc-oxide (IZO), on the overcoat layer 26, and subsequent patterning of the transparent conductive material. Moreover, an organic material, such as polyimide, is deposited on the common electrode 18, and rubbed to form an alignment layer 19.
In FIG. 3F, patterning is performed to form spacers. First, a photosensitive organic material is coated on the entire surface of the alignment layer 19 to form a photosensitive organic layer 28, such that the photosensitive organic material is a negative type that leaves the light-exposure portions after a development process. A mask 30 having light-transmitting portions E and light-shielding portions F is disposed over the photosensitive organic layer 28, such that the light-transmitting portions E correspond to the areas where the desired spacer patterns will be subsequently formed. Then, light L2 irradiates the photosensitive organic layer 28 through the light-transmitting portions E of the mask 30. In FIG. 3G, after a development process is completed, the photosensitive organic layer 28 is developed into spacer patterns 40.
The aforementioned process, however, has some disadvantages. For example, a number of the steps (the photolithographic process) are required to form only spacers, whereby the photolithographic process is more complicated than the spraying method. Further, an additional mask is required to form the spacers during the photolithographic process, thereby increasing production cost.